The package of any electronic device must include contacts for transmitting signals providing power and ground connections between the internal circuitry of the device and external circuitry. Simple examples of prior-art contacts include the wire leads protruding from the ends of a discrete diode or resistor, or the metal caps located on the ends of a fuse. On the other hand, a sophisticated electronic device, such as a microprocessor, may require several hundred contacts. Those devices are usually produced in a package having multiple pins for mounting to a printed circuit board via contact holes.
More modern surface mount techniques can be used to connect a device to a printed circuit board without cumbersome pins. The leads of a surface mount device simply flush mount to the surface of a printed circuit board, such as the motherboard of a personal computer system, onto which the device is attached by soldering to contact wires or conductors. Surface mount leads do not penetrate through the circuit board like a conventional packages having pins, making them efficient to use in production.
Referring now to FIG. 1, a prior-art integrated circuit that is surface mounted to a circuit board is illustrated. This integrated circuit includes a silicon (Si) based integrated circuit 101. Insulating film 102 coats the underside of circuit 101 to protect and passivate it. Epoxy layer 103 and a silicon cap 104 cover circuit 101. Epoxy layer 103 and silicon cap 104 also cover metal bridge 105.
Metal bridge 105 electrically connects circuit 101 to silicon post 106. Epoxy section 111 mechanically secures circuit 101, metal bridge 105 and silicon post 106. Nickel (Ni) plate 107 covers silicon post 106 and forms a butt-joint with metal bridge 105. Nickel plate 107 is electrically coupled to silicon post 106 and metal bridge 105. Nickel plate 107 provides the integrated circuit with a connection point to external circuitry.
This prior-art contact comprises:
1) metal bridge 105, PA1 2) silicon post 106, PA1 3) nickel plate 107, and PA1 4) epoxy section 111.
As illustrated in FIG. 1, the contact of the integrated circuit has been soldered to circuit board conductor 109 with solder fillet 108. Circuit board conductor 109 has been formed onto circuit board substrate 110.
The contact for the integrated circuit illustrated in FIG. 1 provides for various advantages. For example, nickel plate 107 covers the sidewalls of silicon post 106, which helps to strengthen the bond between the circuit 101 and the circuit board substrate 110. This is due to the fact that solder can be placed on nickel plate 107 on the sidewalls of silicon post 106 as illustrated in FIG. 1. It also facilitates inspection during surface mount of the integrated circuit to the circuit board. Whether a good mount is made can be easily confirmed by looking at the solder on the sidewalls of silicon post 106.
Furthermore, nickel plate 107 extends over the sidewalls of silicon post 106 and contacts the side of metal bridge 105, forming a butt-joint interface between nickel plate 107 and metal bridge 105. This provides for an electrical contact between circuit board conductor 109 and circuit 101.
The butt-joint interface of the integrated circuit contact of FIG. 1, however, cannot be formed with much certainty or control over its resulting reliability or bonding adhesion between nickel plate 107 and metal bridge 105. There are a number of reasons for this. The physical surface of the side of metal bridge 105 might not be flat enough to ensure a reliable bond at this butt-joint interface. Furthermore, the side of metal bridge 105 is difficult to clean because of its location on the side of the wafer. The bond at this butt-joint interface therefore might be weakened If the side of metal bridge 105 is not flat or has not been thoroughly cleaned.
The formation of this butt-joint interface also limits the materials that can be used for nickel plate 107 and metal bridge 105. This is so because metal bridge 105 and nickel plate 107 can comprise more than one metal layer. The bonding layer of nickel plate 107 then has to be formed so as to bond with each metal layer at the side of metal bridge 105 in order to form an effective contact. Accordingly, the selection of materials that can be used for metal bridge 105 and for the bonding layer of nickel plate 107 is limited.
FIG. 2 shows a prior-art contact that avoids a butt-joint by using a wrap-around flange contact. Silicon based circuit 101, insulating film 102, epoxy layer 103, silicon cap 104, metal bridge 105, silicon post 106, solder filet 108, circuit board conductor 109, circuit board substrate 110, and epoxy section 111 are similar to that of the above described butt-joint contact. However, wrap around nickel plate 112 and metal bridge 105 have a horizontal flange interface 113. While the wrap around flange avoids the problems associated with a butt-joint, it is still a relatively complex design, requiring a rather involved series of processing steps and a relatively large amount of wafer area dedicated to contact fabrication.